In fill level measurement, microwaves are sent by means of an antenna toward the surface of a fill substance, and the echo waves reflected on the surface are received. The echo waves are preferably plotted as an echo function, from which the travel time is determined. From the travel time, the separation between the surface of the fill substance and the antenna is determined.
All known methods can be applied, which enable measuring of relatively short distances by means of reflected microwaves. The best known examples are pulse radar and frequency modulation continuous wave radar (FMCW radar).
In the case of pulse radar, periodically, broadband microwave transmission pulses, in the following referred to as microwaves, are sent, which are reflected from the surface of the fill substance and after a distance dependent travel time received back. The received signal amplitude plotted as a function of time represents the echo function. Each value of the echo function corresponds to the amplitude of an echo wave reflected at a certain separation from the antenna.
In the case of the FMCW method, a continuous microwave is sent, which is periodically linearly frequency modulated, for example, according to a sawtooth function. The frequency of the received echo signal has, consequently, compared with the instantaneous frequency, which, the transmission signal has at the point in time of receipt, a frequency difference, which depends on the travel time of the echo signal. The frequency difference between transmission signal and received signal, which can be won by mixing the two signals and evaluating the Fourier spectrum of the mixed signal, corresponds, thus, to the separation of the surface of the fill substance from the antenna. Furthermore, the amplitudes of the spectral lines of the frequency spectrum won by the Fourier transformation correspond to the echo amplitudes. This Fourier spectrum represents, consequently, the echo function for this case.
Fill level measuring devices working with microwaves are applied in many branches of industry, e.g. in the chemicals industry and in the foods industry. Typically, the fill level in a container must be measured. The containers usually have an opening, at which a nozzle or, a flange is provided for securement of measuring devices.
Depending on application, usually parabolic-, horn- or rod- or patch antennas are applied in fill level measuring technology. Horn antennas are basically constructed such that a funnel shaped metal horn is formed on a hollow conductor in the direction facing the fill substance. The construction of a parabolic antenna can be described in simple manner that the microwaves are guided in a hollow conductor and radiated out, and/or coupled back in, in the focal point of the parabolic mirror directly or by means of a reflector. A rod antenna is composed basically of a hollow conductor, which is filled at least partially with a rod of a dielectric and which has in the direction facing the fill substance a coupling structure in the shape of a taper or a cone. These three freely radiating antenna types are usually fed via a coaxial cable, which is connected to an exciter element protruding into the hollow conductor.
If one, selects a horn antenna with a relatively large exit opening, less divergent signal fractions result. One speaks of a focusing in a radiated direction. The conventional measure for the focusing is the so-called “3 dB lobe width”. This tells at which angle of the radiated, respectively received, power fraction of the microwaves has declined to exactly half of the maximum value in the radiated direction. If one selects a relatively large exit opening of the horn antenna, the length of the horn antenna must be correspondingly matched, in order to avoid so-called “side lobes”. Side lobes are other maxima of the power fractions of the microwaves, which are not directed in the radiated direction.
Antenna arrangements for fill level measurement have the goal of achieving a large focusing effect. This means focusing more power fractions with targeting in a radiated direction. This is especially advantageous in the case of very large distances between the antenna arrangement and the medium in the range of 30 . . . 80 m, as well as in the case of surfaces of the medium, which reflect back only a small power fraction into the antenna. Reasons for a small fraction of back reflected power fractions can include a small dielectric constant of the medium, in which the microwaves are transmitted, absorption in the case of bulk goods, as well as a wavy surface (for example, because a stirrer is present), whereby power fractions are reflected back in other directions than in the direction of the antenna arrangement.
If a stirrer is arranged in the container, which leads to deflection of the microwaves in other directions than the radiated direction, this effect can be weakened with a horn antenna having a smaller focusing effect. Moreover, there is, for reasons of cost, interest in horn antennas with smaller horn diameters, especially in the case of horn antennas of stainless steel. Since the mounting usually takes place outside of the container, the maximum possible horn diameter is further fixed by the flange diameter, since the horn in the case of this mounting must pass through this.
European application, EP 1 485 683 B1 describes a horn antenna for a radar, fill-level measuring device for determining a fill level of a medium in a container. The horn antenna includes a first conductive housing, whose inner space is filled with a dielectric body. A second conductive housing lengthens the first housing in radiated direction of the microwaves.
Since the second housing of the horn antenna is electrically conductively and is electrically connected with the first housing, after a certain fill level of the medium in the container, an electrically conductive connection between the horn antenna and the medium can arise, along with an electrically conductive connection between the horn antenna and the container. This can lead to a so-called “ground loop”, as well as also to short circuits and explosions.